According to the NPD Group, 30 percent of US buyers who bought an iPhone 3G from June to August 2008 switched from other carriers. 47 percent of the switchers came from Verizon, 24 percent from T-Mobile and 19 percent from Sprint (boy, you Sprint folks are loyal, or do you just love the Samsung Instinct that much?).

Before the launch of the iPhone 3G, the iPhone had 11 percent of the US smartphone market. Now it's up to 17 percent-- quite impressive.

Here's NPD's full press release with all the details:

THE NPD GROUP: ONE IN THREE IPHONE 3G BUYERS SWITCHED FROM OTHER CARRIERS TO JOIN AT&T

Apple iPhone 3G now the second best-selling mobile phone handset among U.S. consumers, after Motorola's RAZR V3

PORT WASHINGTON, NEW YORK, OCTOBER 6, 2008 -- According to a new report from The NPD Group, the leader in market research for the wireless industry, 30 percent of U.S. consumers who purchased Apple's new iPhone 3G from June through August 2008 switched from other mobile carriers to join AT&T, the exclusive mobile carrier for the iPhone in the U.S. By way of comparison just 23 percent of consumers, on average, switched carriers between June and August 2008. Nearly half (47 percent) of new AT&T iPhone customers that switched carriers switched from Verizon Wireless, another 24 percent switched from T-Mobile, and 19 percent switched from Sprint.

"The launch of the lower-priced iPhone 3G was a boon to overall consumer smartphone sales," according to Ross Rubin, director of industry analysis for The NPD Group. "While the original iPhone also helped win customers for AT&T, the faster network speeds of the iPhone 3G has proven more appealing to customers that already had access to a 3G network."

According to NPD's "iPhone 3G Report," before the launch of the iPhone 3G, iPhone sales represented 11 percent of the consumer market for smartphones (January through May 2008); however, after the launch of iPhone 3G, Apple commanded 17 percent of the smartphone market (January through August 2008).

The average price of a smartphone sold between June and August 2008 was $174, down 26 percent from $236 during the same period last year. During June through August 2008, the top four best-selling smartphones based on unit-sales to consumers were as follows:

Methodology: The NPD Group compiles and analyzes mobile device sales data based on more than 150,000 completed online consumer research surveys each month. Surveys are based on a nationally balanced and demographically-representative sample, and results are projected to represent the entire population of U.S. consumers. Note: Sales figures do not include corporate/enterprise mobile phone sales. For more information about NPD's offerings in this business sector, visit http://wireless.npd.com.

Since the merger with Nextel, Sprint has a fairly large customer base (3rd in the nation behind ATT and Verizon). Sprint has many more customers than T-Mobile, but the percentage of T-Mo customers switching was nonetheless higher, according to NPD.